So-called "brushless D.C." motors are actually permanent magnet field polyphase synchronous A.C. motors which are not being driven by sinusoidal A.C. waveform excitation. Because of cost, space and power dissipation requirements, most brushless D.C. motors are driven by discontinuous, "square wave switched" waveforms which at best crudely approximate sinusoidal driving waveforms. The abrupt step function switching of the driving signals are readily generated with digital switching transistor drivers which have minimized heat dissipation (which occurs only during transitional edges of the waveform). However, the use of digital driving signals causes very large rates of change of motor winding currents and/or voltages and results in radiation of A.C. fields from the windings and interconnecting leads and driver electronics with much higher frequency components than the fundamental of the desired sinusoidal waveform. These stray A.C. fields result in electrical noise and interference. In disk drives, for example, low level read channel flux transition sense preamplifiers may be in physically close proximity to the brushless D.C. spindle motor, and the resultant electrical noise may seriously degrade preamplifier performance.
In addition, when driving currents are abruptly switched between motor windings rather than having them change sinusoidally, the point of application of the generated motor torque upon the iron path and windings of the motor abruptly changes; and, these abruptly changed forces cause deformation and vibration that results in very objectionable acoustic noise, particularly for a personal computer or desktop business machine location at which the user is typically in close proximity.
Both full wave and half wave center-tapped or bipolar and unipolar driver and winding connections are used with brushless D.C. motors. In the presently preferred arrangement, a full wave driving configuration is employed during an initial, power-on interval when the spindle motor is spinning up toward its desired operating speed. Once about half the operating speed is reached, a half wave, center-tapped connection is implemented, and this configuration will be discussed in connection with the prior art and with the new circuit design, although the principles of the present invention are readily applicable to, and are easily extended to, full wave or bipolar configurations as may be desired.
With reference to FIGS. 1 and 2, the commutation phase angle signals for a conventional brushless D.C. motor 11 are obtained either with the aid of some type of position encoder 12 (such as a magnetic encoder employing Hall-effect semiconductor devices), or with the aid of voltage waveform comparators which monitor back EMF induced in the motor windings in so-called "Hall-less" brushless D.C. motors. As shown in FIG. 1, the encoder 12 and suitable downstream processing circuitry produce square wave signals HA, HB and HC (shown in FIG. 2 in relationship to the near sinusoidal motor terminal phase voltages A, B, and C with respect to the neutral or center-tap N of a Y-connected rotating three phase brushless D.C. motor, such as the motor 11 shown in FIG. 1).
Conventional excitation current waveforms are shown in FIGS. 1 and 2 as I.sub.A, I.sub.B and I.sub.C, with current flow shown in FIG. 1 by arrows associated with each current I. The abrupt edges of these conventional driving signals cause the objectionable electrical and acoustical noise to be generated within the motor 11. It is a common expedient of the prior art to add low pass filters or slew rate limiters of some form to the drivers of these motor systems to produce the dotted waveform segments shown rounding slightly the corners of the step function driving current waveforms I.sub.A, I.sub.B and I.sub.C as shown in FIG. 2.
While these prior techniques can be useful to reduce the electrical noise to acceptable levels by attenuating the higher frequency harmonics, they do not adequately solve the acoustic noise problem. While a rate of change of current which is on the order of the inductance to resistance (L/R) time constant of the motor windings (typically a few tens of microseconds) makes the electrical noise acceptably low, this rate of change is still too fast to reduce adequately the acoustic noise. When the rates of change of the motor driving currents are made much longer by known methods employed in the prior art, one or more of a number of difficulties typically arises. First, the effective commutation point at which currents in successive phases reach equal magnitude is delayed so far that motor torque output is significantly reduced, thereby greatly impairing motor efficiency. Second, the start of the switching can be advanced to account for the delay of the switching point by providing considerable additional electronics at greater overall system complexity and cost. However, if the rate is not adjusted to track the amplitude of the current, then the commutation point still varies. This is a problem with many active linear ramp generation circuits. Third, capacitor values of the required low frequency filters become physically bulky and expensive as to be impractical for many small sized applications, such as micro-Winchester and smaller disk drives. Fourth, locating the filter in the active circuitry of the driver achieves a size reduction but results in sensitivity to active device parameter variation tolerances that shift the effective commutation point. Fifth, if the waveforms are exponential functions of time, as is typical of passive filters, they greatly deviate from a sinewave or even a trapezoidal waveform at the edges. Sixth, the sum of turning-on and turning-off waveforms should be constant and equal to the command value to prevent commutation point shift and torque ripple; yet this requirement is very difficult to achieve with filters.
Thus, a hitherto unsolved need has remained for a driver circuit which reduces noise in a brushless D.C. motor without the attendant drawbacks and disadvantages of the prior art as noted hereinabove.